1. Field of the Invention
This invention relates to a technique for the injection molding of removable sections in polymeric articles.
2. Description of the Prior Art
Junction boxes used for making electrical connections typically have circular knockout sections for the optional installation of wiring into the boxes. When the junction boxes are made of metal, the knockout sections are typically made by stamping a pattern in the sheet metal that is used to form the box. In many cases, this results in gaps around the periphery of the knockout piece being supported at one or two points along its edge. Many types of junction boxes, including pedestals used for the outdoor installation of telephone cable, are made of plastic or other polymeric materials. In many cases, it is desirable to maintain a hermetic seal in such enclosures; therefore, any knockout portions that have not been removed must not have gaps around the edges. The knockout portions should be strong enough to prevent inadvertent leakage prior to the removal of the knockout, but the knockout should also be capable of being removed conveniently, for example with a hammer or other simple tool, without damage to the rest of the enclosure.
In the past, one method by which knockout regions have been provided in polymer materials is by first producing a hole of the desired size and plugging it with a grommet to provide a seal until removed. This requires more time and expense than if the knockout were formed directly in the polymer material. However, it is typically not feasible to employ stamping techniques to form a knockout region in polymeric material, especially if it is desired to maintain a hermetic seal prior to removal of the knockout.
One of the more common molding techniques utilized with polymeric material is injection molding, including reaction injection molding, whereby fluid polymer or prepolymer material comprising one or more fluid components is injected under pressure into a cavity having the desired shape. However, restrictions of the flow of the material during injection can produce gaps, voids, and other undesirable effects in the molded article. It is desirable to have a technique for forming a knockout section in an injection molded article that satisfies the above criteria of maintaining a hermetic seal until removed, while allowing ease of removal, and maintaining compatibility with the injection molding process.